January 22, 2004: On January 24, at about 9:05 p.m. Pacific
Standard Time, NASA's second rover is scheduled to arrive on Mars.
Opportunity will land near the equator, on a plain known as Meridiani
Planum. It'll be halfway around the planet from Gusev Crater, where
its twin, Spirit, is already feeding eager scientists as much data
as it can transmit.

Meridiani
Planum interests scientists because it contains an ancient layer of
hematite, an iron oxide that, on Earth, almost always forms in an
environment containing liquid water. The site appears dry now. So
how did the hematite get there? Was there once water in the area?
If so, where did it go?

"There are five or six hypotheses to explain the hematite on
Mars, but none of them are a slam-dunk," says NASA's Mars Landing
Site Science Coordinator John Grant. "We have to go there to
find out which is correct."

It's possible, for example, that the hematite was produced directly
from iron-rich lavas, a process that would not require liquid water.
But if water was involved--and that's considered most probable--then,
most likely, the hematite either formed from the iron-rich waters of
an ancient lake, or it formed when Martian groundwater percolated though
layers of volcanic ash.

Opportunity's suite of spectrometers, cameras, microscopes, and sampling
tools should allow scientists to figure out where the hematite came
from. For instance, if a mineral called goethite is found among the
hematite, that would mean that the hematite formed in watery conditions.
On the other hand, if magnetite is found and goethite is not, a watery
past is unlikely.

Just being able to look at the way the hematite is distributed will
provide some answers. If the hematite occurs as a thin layer within
a pile of layers, then it's likely to have formed in a long-ago lake,
says Grant. If, on the other hand, it occurs in more discrete veins,
deposited between cracks in rocks, "then it's much more likely
to have been associated with groundwater."

If you look in the Earth, he says, in places where the groundwater
percolates through the subsurface, "you see evidence for life
all over the place." This mission, Grant emphasizes, is not seeking
evidence of Martian life. It's looking for environments that were
favorable for life, and in which evidence of life may have been preserved.

Knowing how the hematite formed will help determine if Meridiani
Planum is that kind of environment.

Meridiani
Planum is unique on Mars because there's so much exposed hematite
there, according to data gathered by NASA's Mars Global Surveyor spacecraft.
"Localized deposits also exist in two other sites: the deep canyon
Valles Marinaris and a place called called Aram Chaos," notes
Grant, "but neither are accessible based on the current landing
system." Meridiani Planum has more hematite and it's a safer
place to land.

Right:
An artist's concept of ancient hot springs on Mars where hematite
might have collected. [more]

Meridiani
Planum is also attractive because the site appears to be eroding,
with once-buried craters that are now half-revealed. Opportunity might
be able to inspect layers of ground that would otherwise be hidden,
affording a glimpse into the area's distant past.

"There's so much we don't know about Mars," says Grant.
"But I really think we're going to come out of this mission with
a better understanding of what Mars has been like over time, and where
we might go for our next step."